`Self` with type aliases could emit a better error

[lcnr]

type Foo<T>
where
    Self: Clone,
= Vec<T>;

results in

error[E0411]: cannot find type `Self` in this scope
 --> src/lib.rs:3:5
  |
3 |     Self: Clone,
  |     ^^^^ `Self` is only available in impls, traits, and type definitions

it may makes sense to add a "forbidden" rib to type aliases which contains Self and emits a better error for this or at least adds a note like "Self cannot be used to refer to type aliases"